Abstract : A universal receiver having a pneumatic safe/arm/firing mechanism is provided. The receiver comprises a receiver shell with a threaded barrel clampnut on one end and a threaded breech plug assembly on the opposite end. The breech plug assembly includes a cocking and striker piece and a pneumatically-operated sear. A separate pneumatically-operated safety bar interlocks with the cocking and striker piece to prevent inadvertent firing. A multi-port pneumatic valve is connected to sequentially operate the safety bar retractor followed by the sear retraction, thereby firing the gun.